ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-12-10239-2012Mixing state and compositional effects on CCN activity and droplet growth kinetics of size-resolved CCN in an urban environmentPadróL. T.13MooreR. H.1ZhangX.2RastogiN.24WeberR. J.2NenesA.121School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA3present address: Department of Civil and Environmental Engineering, Tufts University, Medford, MA, USA4present address: Physical Research Laboratory, Ahmedabad, India0611201212211023910255This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/12/10239/2012/acp-12-10239-2012.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/10239/2012/acp-12-10239-2012.pdf

Aerosol composition and mixing state near anthropogenic sources can be highly
variable and can challenge predictions of cloud condensation nuclei (CCN).
The impacts of chemical composition on CCN activation kinetics is also an
important, but largely unknown, aspect of cloud droplet formation. Towards
this, we present in-situ size-resolved CCN measurements carried out during
the 2008 summertime August Mini Intensive Gas and Aerosol Study (AMIGAS)
campaign in Atlanta, GA. Aerosol chemical composition was measured by two
particle-into-liquid samplers measuring water-soluble inorganic ions and
total water-soluble organic carbon. Size-resolved CCN data were collected
using the Scanning Mobility CCN Analysis (SMCA) method and were used to
obtain characteristic aerosol hygroscopicity distributions, whose breadth
reflects the aerosol compositional variability and mixing state. Knowledge of
aerosol mixing state is important for accurate predictions of CCN
concentrations and that the influence of an externally-mixed, CCN-active
aerosol fraction varies with size from 31% for particle diameters less
than 40 nm to 93% for accumulation mode aerosol during the day.
Assuming size-dependent aerosol mixing state and size-invariant chemical
composition decreases the average CCN concentration overprediction (for all
but one mixing state and chemical composition scenario considered) from over
190–240% to less than 20%. CCN activity is parameterized using a
single hygroscopicity parameter, κ, which averages to
0.16 ± 0.07 for 80 nm particles and exhibits considerable variability
(from 0.03 to 0.48) throughout the study period. Particles in the 60–100 nm
range exhibited similar hygroscopicity, with a κ range for 60 nm
between 0.06–0.076 (mean of 0.18 ± 0.09). Smaller particles (40 nm)
had on average greater κ, with a range of 0.20–0.92 (mean of
0.3 ± 0.12). Analysis of the droplet activation kinetics of the aerosol
sampled suggests that most of the CCN activate as rapidly as calibration
aerosol, suggesting that aerosol composition exhibits a minor (if any) impact
on CCN activation kinetics.